Re: Make EXPLAIN generate a generic plan for a parameterized query

On Wed, 2022-10-12 at 00:03 +0800, Julien Rouhaud wrote:
> On Tue, Oct 11, 2022 at 09:49:14AM -0400, Tom Lane wrote:
> > I think it might be better to drive it off an explicit EXPLAIN option,
> > perhaps
> >
> > EXPLAIN (GENERIC_PLAN) SELECT * FROM tab WHERE col = $1;
> > 
> > If you're trying to investigate custom-plan behavior, then you
> > need to supply concrete parameter values somewhere, so I think
> > this approach is fine for that case.  (Shoehorning parameter
> > values into EXPLAIN options seems like it'd be a bit much.)
> > However, investigating generic-plan behavior this way is tedious,
> > since you have to invent irrelevant parameter values, plus mess
> > with plan_cache_mode or else run the explain half a dozen times.
> > So I can get behind having a more convenient way for that.
> 
> One common use case is tools identifying a slow query using pg_stat_statements,
> identifying some missing indexes and then wanting to check whether the index
> should be useful using some hypothetical index.
> 
> FTR I'm working on such a project and for now we have to go to great lengths
> trying to "unjumble" such queries, so having a way to easily get the answer for
> a generic plan would be great.

Thanks for the suggestions and the encouragement.  Here is a patch that
implements it with an EXPLAIN option named GENERIC_PLAN.

Yours,
Laurenz Albe